Multiple methods as well as respective devices for measuring the refractive index of samples are known in the state of the art. Especially refractometry is used for measurement of the refractive index. For instance an Abbe refractometer can be used to measure the refractive index of a sample. Such a refractometer comprises a prism adjoining a sample and a light source being arranged to generate a beam of light travelling through the prism to the sample whereas one part of the beam is diffracted and another part of the beam is reflected at the prism-sample interface. This behavior can be described by Fresnel's law. The reflected part of the beam may be detected by means of a light detector. Above a critical angle of incidence that depends on the refractive index of the sample, the light is totally reflected. At the critical angle, the intensity of the reflected light changes substantially resulting in an intensity profile recorded by a light detector allowing thus an evaluation of the sample's refractive index. This general principle is known to the person skilled in the art for a long time. Furthermore, other methods are known for measuring the refractive index, as for example by means of a goniometric refractometer or ellipsometry.
The knowledge of the refractive index of a sample may result in important information on the properties of a sample. For instance, refractive index measurement is frequently used for the determination of alcohol or sugar concentrations.
Another interesting property for characterization of samples may be the temperature coefficient of the refractive index.
Document WO 91/17425 discloses an apparatus for analyzing optical properties of transparent objects comprising a light source and an analyzer cell having a cavity formed therein for receiving a sample. A beam of light is generated by the light source guided through the analyzer cell and the sample. Preferably an optical fiber is used as heat source. Furthermore, this document mentions, although not in detail, a method of determining the temperature coefficient of the refractive index of a fluid, basically comprising the steps of providing a cell with a fluid, creating a known (spatial) temperature gradient in the fluid between known points in a given plain, analyzing the refractive index profile of the fluid in said plane, and determining the temperature gradient coefficient of the refractive index of the fluid from the analyzed refractive index profile and known temperature gradient. Unfortunately, this method can only be used for liquid samples. Furthermore, the method is unsuitable for samples underlying a chemical reaction which necessitates a constant temperature extending over the whole sample. Especially in the case of chemical reactions it is often undesirable to have different temperature across the sample because this might strongly influence the complete reaction and/or might induce convective flow.
U.S. Pat. No. 6,970,256 B1 discloses another apparatus for measuring the thickness and refractive index of a sample. The apparatus comprises a prism mounted in a prism support, a sample in contact with one side of the prism and thermal elements for heating the sample and/or prism to a specific temperature. For the sake of determination of the refractive index, light is emitted by a light source at the prism sample interface at a certain angle depending on the refractive index, whereas reflected light is detected by a detector. Additionally it is mentioned, but not described in detail, that the temperature coefficient of the refractive index could be measured for films and bulk materials by measuring the index at temperatures other than room temperature, basically due to the possibility of adjusting temperature by means of the thermal elements.
All known methods for measuring the temperature coefficient of the refractive index have the disadvantage that they do not allow for measurement of temporally developing samples. Time or frequency dependent processes cannot be analyzed by the above mentioned methods. Furthermore, the above methods are unsuitable or do not describe how the temperature coefficient index could be measured in course of phase transitions of samples, or cannot be used to analyze structural formation processes.
Thus, the technical problem might be to overcome at least one of the above mentioned disadvantages or to provide an advanced method for determining the temperature coefficient of the refractive index, in particular for analyzing dynamic properties of samples.